1. Field of the Invention
The present invention relates to prevention of illegal copies in image forming and the like.
2. Description of Prior Art
Recently electrophotographic copying machines become popular, and anyone can use them easily to reproduce characters and images printed on a sheet of paper or the like. Especially, image data processing and image forming technologies in a digital copying machine, a multifunctional peripheral or the like are improved very much, and even a copy very difficult to be distinguished from its original can be produced easily.
Therefore, an original such as an important document or a secret document whose reproduction should be inhibited can be reproduced to provide copies substantially the same as the original illegally and the copies can be leaked outside. Further, scanning apparatuses also become popular, and an original such as an important document or a secret document inhibited to be reproduced can be scanned and transmitted via a network as an electronic file. Therefore, it is an important problem to prevent forgery, falsification, illegal copying, and illegal reproduction of important documents and the like.
As a countermeasure for preventing illegal copying, it is proposed to embed a latent image not recognized or difficult to be recognized with naked eye in an image of an original and to visualize the latent image when the original is copied (for example, Japanese Patent laid open Publication 2004-223854). This has an effect to prevent illegal copying psychologically because a copy not equivalent to an original is produced when the original includes such a latent image. In this technology, the latent image embedded in an image of the original is an additional image not recognized or difficult to be recognized with naked eye, and it is prepared to become visualized apparently when the original is reproduced with an image forming apparatus. For example, if characters of a character of “COPY INHIBIT” are embedded in an image of an original, when a printed material of the original including the additional image is copied, the additional image embedded in the image, that is, the hidden characters of “COPY INHIBIT” are visualized. Therefore, a copy equivalent to the printed material is not produced, and when one observes the copy, the characters of “COPY INHIBIT” not included in the original are noticed. Therefore, the copy is recognized easily that it is not an original. Thus, illegal copying is prevented psychologically.
However, when an original embedding the above-mentioned latent image for preventing illegal copying psychologically is printed with an image forming apparatus to provide a copy, the additional image may not be formed as intended on a sheet of paper. For example, when a printed material including a latent or additional image is enlarged or reduced, the visualized additional image is also enlarged or reduced. When the additional image is subjected to enlargement copy, it may become defective, or only a part of the additional image may be reproduced. When the additional image is subjected to reduction copy, it may not be visualized apparently. In such a case, the effect to suppress illegal copying psychologically is damaged. Further, it may be difficult to distinguish the copy from the original.
As will be explained later, a code is embedded in a background in this embodiment, and this can be considered as a kind of electronic watermark. In an apparatus disclosed in Japanese Patent laid open Publication 2003-169204, the embedded electronic watermark is detected when the image is scaled or rotated. It cannot be distinguished in this technology whether the image is a copy or not.